Elements taking advantage of a Peltier effect or Seebeck effect are used as thermoelectric conversion elements. Since thermoelectric conversion elements have a simple structure, are easy to handle and able to maintain a stable characteristic, widespread use of thermoelectric conversion elements is attracting attention in recent years. Especially when used as an electronic cooling element, the thermoelectric conversion element can perform local cooling and accurate control over temperature close to a room temperature, and therefore a wide range of studies are being carried forward aiming at temperature stabilization of opto-electronics and semiconductor laser or the like.
The aforementioned electronic cooling element or thermoelectric module used for thermoelectric power generation is configured as shown in FIG. 7 by connecting p-type thermoelectric conversion element (p-type semiconductor) 5 and n-type thermoelectric conversion element (n-type semiconductor) 6 via a connection electrode (metal electrode) 7 to form a pn element pair and arranging a plurality of such pn element pairs in series. At this time, depending on the direction of a current flowing through each pn element pair, one end of p-type thermoelectric conversion element 5 and n-type thermoelectric conversion element 6 is heated and the other end is cooled. In FIG. 7, reference numerals 8 and 9 denote external connection terminals, 10 denotes a ceramic substrate and H denotes an arrow indicating a heat flow direction.
For the material of this thermoelectric conversion element, a material having large performance index Z (=α2/ρK) expressed by Seebeck coefficient α which is a substance-specific constant, specific resistance ρ and thermal conductivity K is used in the temperature region where the element is used. Crystal materials generally used as thermoelectric conversion elements are Bi2Te3-based materials and these crystals have an outstanding cleavage property and these crystals are known to have a problem that after undergoing slicing and dicing steps or the like to obtain a thermoelectric conversion element from an ingot, the yield becomes extremely small due to cracking or chipping.
To solve this problem, a method for manufacturing a thermoelectric conversion element module is being tried, which undergoes various steps such as a heating step of mixing material powders so as to have a desired composition and heating/melting the mixture, a coagulation step of forming a solid solution ingot of a thermoelectrically converted material having a rhombohedral structure (hexagonal structure), a crushing step of cursing the solid solution ingot and forming solid solution powder, a sizing step of uniformalizing the grain size of the solid solution powder, a sintering step of sintering the solid solution powder of the uniformalized grain size under a pressure, and a hot upset forging step of making the powder sintered substance plastic-deformed in hot pressing and rolling and thereby orienting crystal grains of a powder sintered structure in a crystal orientation of an excellent performance index or the like (e.g., see Patent Literature 1).
Furthermore, as a conventional method for manufacturing a thermoelectric conversion element module, a manufacturing method is known, which includes a step of manufacturing an alloy ingot, a crushing step of crushing the alloy ingot under a vacuum with an oxygen concentration of 100 ppm or below or under an atmosphere of inert gas into raw powder having an average powder grain size of 0.1 μm or above and 1 μm or below and a sintering step of sintering the raw powder through electric resistance heating while adding a pressure to the raw powder. In the sintering step, a pulse-shaped current is made to flow, the raw powder is sintered with its joule heat and a pressure of 100 kg/cm2 or above and 1,000 kg/cm2 or below (9.8 MPa or above and 98.1 MPa or below) is added to the raw powder during the sintering. This manufacturing method allows a thermoelectric conversion material of a fine crystal grain size and with excellent workability to be manufactured (e.g., see Patent Literature 2).
Furthermore, as a conventional method for manufacturing a thermoelectric conversion element module, a method is known whereby an entire tube made of a heat-resisting insulating material is accommodated in a crucible that accommodates a molten thermoelectric conversion material and the molten thermoelectric conversion material is filled into the tube under a pressure (e.g., see Patent Literatures 3 to 6). Furthermore, as a method for manufacturing a thermoelectric conversion element, a method is known whereby one end of a glass capillary is inserted into a molten thermoelectric conversion material, the thermoelectric conversion material is sucked up, the thermoelectric conversion material is coagulated, the capillary is cut and a thermoelectric conversion element is thereby obtained (e.g., see Patent Literature 7).